1. Field of the Invention
This invention relates to a radio system that divides its communication area into two or more cells to permit frequency reuse among the cells. In particular, it relates to a radio system for use in communications providing high-speed communication service such as data and image transmission.
2. Description of the Related Art
There are some conventional wireless subscriber lines called WLL (Wireless Local Loop) and FWA (Fixed Wireless Access), LMDS (Local Multipoint Distribution Service). In such subscriber lines, one-to-multi directional radio equipment, called a P-MP (Point-Multi Point) system, is constituted of wireless transmission lines that connect base stations installed by a telecommunications carrier with many subscriber stations installed at users premises.
This system installs two or more base stations in a certain region to provide service in all the parts of the region. In this system, however, frequency reuse is needed for effective utilization of limited frequency resources. Such frequency reuse is common in mobile communication technology for mobile and cellular phones. To prevent the occurrence of interference, the system is also required not only to use different frequencies among all sectors, but to avoid use of the same frequency in adjacent base stations as well.
The above-mentioned P-MP system uses high frequencies of submillimeter or millimeter wave bands to provide communication through the air over line-of-sight distances, which is based on the ARIBSTD-T59 standard. According to the standard, frequency bands of 26 or 38 GHz has to be used, together with the use of high-gain antennas of 20 dBi or more at subscriber stations.
A cell configuration in a conventional P-MP system is disclosed in Japanese Patent Application Laid-open No. 10-042352 entitled. Radio System. (Applicant: Mitsubishi Electric Corp., Inventor: Koichi Ishii) published on Feb. 13, 1998. A typical sector configuration will be described below.
FIG. 10 is a plan view of a conventional layout of base stations, showing a case when subscriber stations face each other and thereby cause interference.
In FIG. 10, B1 to B6 are base stations and F1 to F3 are frequencies F1 to F3 used at each antenna, indicating each base-station area in the form of a hexagon for convenience sake. This configuration is effective in an FDD system using different transmit frequencies between the base stations and terminal stations (subscriber stations).
On the other hand, a TDD system using the same transmit frequencies between the base stations and the terminal stations may cause some subscriber stations to face each other. In this case, great interference occurs and high-quality service cannot be offered.
In general, the P-MP system is not designed for synchronization between the base stations. Therefore, in FIG. 10, interference occurs between subscriber stations C1 and C2 (which commonly use frequency F3) and subscriber stations C3 and C4 (which commonly use frequency F2).
If there is no synchronization between sectors, interference also occurs between subscriber stations C5 and C6 (which commonly use frequency F2) within the same base-station area.
Description will be made next about the above-mentioned problems with reference to FIGS. 10 through 12, taking as an example the conventional case of interference between subscriber stations C1 and C2.
FIG. 11 is an elevation view showing the position of a base station and subscriber stations for calculating the amount of interference in the conventional layout of the base stations.
Suppose that transmission power of the base station and the subscriber stations is 17 dBm, the antenna gain at the base station is 15 dBi, and the antenna gain at the subscriber stations is 30 dBi.
In general, a parabola antenna with a gain of 30 dBi radiates a beam of about three to four degrees in width.
Suppose further that the service area of the base station is 1 km, the distance of the base station B1 to the subscriber station C1, and the base station B6 to the subscriber station C2 is 1 km, and the distance of the subscriber stations C1 and C2 is 8 km.
In addition, installation of antennas are made the same in height between the base stations B1 and B6, and between the subscriber stations C1 and C2, respectively, for the sake of simplicity.
The elevation angle of the antenna that the subscriber station allows for the base station depends on the difference of elevation between the base station and the subscriber station. If the difference of elevation is 30 m, the elevation angle will be about 1.7 deg. This elevation angle is just one-half the beam width of the antenna or less, and therefore, the gain of the antenna at the subscriber station C2 toward the subscriber station C1 is made only about 3 dB lower than the maximum gain.
The level of a desired wave to be transmitted from the base station B1 and reach the subscriber station C1 can be determined by the following equation: transmission output+transmitting antenna gain−on-air attenuation+receiving antenna gain.
That is, 17+15.120+30=−58 dBm.
On the other hand, the level of an interference wave to be transmitted from the subscriber station C2 and reach the subscriber station C1 would be 17+27.139+27=−68 dBm.
It is preferable, though it depends on the apparatus used, that the ratio of the desired wave to the interference wave (DU ratio:Duty Factor Ratio) is at least 20 dB or more. However, the above DU ratio calculated is just 10 dB and apparently insufficient for high-quality communication.
It is also easy to understand that, since interference between the subscriber stations C3 and C4, and between the subscriber stations C5 and C6 occurs at a distance closer to the occurrence of interference between the subscriber stations C1 and C2, the levels of these interference waves become greater.
FIG. 12 is a plan view showing a conventional layout of a base station, indicating a case when subscriber stations face each other and thereby cause interference.
As shown in FIG. 12, if the number of sectors at each base station is four, subscriber stations may face each other between adjacent cells. In this case, the interference also becomes greater because the subscriber stations C1 and C2 are located close to each other.
It is hard for the conventional radio systems to prevent the occurrence of interference between subscriber stations facing each other. The position of the subscriber stations facing each other may not cause a big problem in an FDD (Frequency Division Duplex) system that uses different frequencies for transmission and reception. However, it may cause a big problem in a TDD (Time Division Duplex) system that uses the same frequency for transmission and reception. For example, if the subscriber stations communicating at the same frequency face each other in the TDD system, a serious interference problem will occur.